fix computation of chunk lines in current_line() so that chunk header and footer are not included

- if chunk has length 1, returned vector has only one instead of 2 elements
- if (and only if) chunk has length 0, line with chunk header is reported

Tested with the following file:

    Test.

    ```{r echo=F}
   options(knitr.package.verbose=T)
   opts_chunk$set()
   ```

    Bar.

    ```{r}
   knitr:::current_lines(4)
   knitr:::current_lines(5)
   knitr:::current_lines(6)
   ```
   ```{r}
   3
   ```

    Baz.

Replaces #691.